(Photo: Gray Jay by GKS)
As the first big snows of the season blanket the high country in Washington and Oregon, many people are cheerfully waxing their skis and snowboards and are rummaging through closets to find their fuzzy hats and gloves. For many animals living in the Cascade Range and Olympic Mountains, the arrival of snow and ice is a far less happy time.
What is the Problem with Winter?
Mammals and birds maintain body temperatures well above the temperature of their surroundings. To do this, they must feed regularly, fueling the fires of their metabolisms. When air temperatures are very cold, extra food is needed to maintain a warm body temperature. This is a major problem because food is hard to come by in winter. Most mountain plants either die or go dormant in winter, which means that herbivorous animals are faced with the possibility of starvation in that season. Carnivores might have better luck finding food in winter, because they can prey on the weakened herbivores and eat carrion.
Over millions of years, a variety of strategies for enduring brutal winter conditions have evolved among mountain-dwelling animals. Warm-blooded (i.e. endothermic) animals have adaptations such as thick fur or downy feathers that help them retain body heat. But fur and feathers aren’t much good when there is no food to generate body heat in the first place. Most of the survival strategies described below are physiological and behavioral tricks used by animals to deal with the seasonal scarcity of food.
Strategy 1: Get the Heck Out of the Mountains
Perhaps the simplest winter survival strategy used by animals is migration. Why suffer through frigid snow storms when you can just go somewhere warmer?
There are two main types of seasonal migration: latitudinal migration and altitudinal migration. Highly mobile animals, such as birds and bats, can migrate hundreds or thousands of miles to the south, where winters are warm and food is plentiful. This is latitudinal migration.
In altitudinal migration, animals move to lower elevations for the winter. Elk, deer, mountain goats, and some birds such as the varied thrush are altitudinal migrants.
Strategy 2: Slow Down and Chill Out
If a high metabolism is costly for animals to maintain in winter, one option is to lower body temperature and reduce movement so that less food is needed to stay alive. Hibernation is a physiological condition lasting weeks or months, in which an animal’s metabolism slows down and its body temperature drops.
Bears do not really hibernate– they just sleep deeply in winter. Their body temperatures do not drop dramatically. Bears and some true hibernators eat like crazy in the months when they are active, to pack on the fat and muscle they need to survive the winter. Some animals that hibernate don’t fatten up as much, but they do stock up on food, such as seeds, that they can eat in their winter burrows when they periodically wake up.
Only a handful of northwest mammals are true hibernators. The best examples are some members of the squirrel family: marmots (yellow-bellied, hoary, and Olympic), ground squirrels (Townsend’s and California), and chipmunks (Townsend’s).
Cold-blooded (i.e. ecotothermic) animals such as reptiles, amphibians, and insects can’t maintain body temperatures higher than their surroundings, so they generally have no choice but to hibernate. Their bodies cool down and they become inactive. They must do what they can to keep from freezing, however, because ice crystals cause death by destroying cells.
Amphibians such as the cascades frog and long-toed salamander burrow in the sediment at the edges of lakes– underwater– where the temperature stays just above freezing all winter. They don’t drown because their metabolisms are so low; they absorb all the oxygen they need through their permeable skins.
Insects are cold-blooded and often hibernate in winter. Ladybugs (i.e. ladybird beetles), for example, gather by the thousands to hibernate in deep rock crevices of the Cascades. Hibernation in insects is often called diapause, and occurs in different life stages– egg, larva, adult– depending on the species.
Strategy 3: Reproduce, Then Die
Like annual plants, some invertebrates don’t survive the winter at all. They feed and reproduce in the spring and summer and are dead by the time the ice and snow arrives. They leave behind their offspring, in the form of either eggs or larvae, which hibernate (i.e. remain in diapause) until warm weather returns. Aphids, mosquitoes, and grasshoppers use this strategy.
Strategy 4: Stay home and Stay Awake
Many mammals and birds stay up in the mountains all winter and carry on fully active lifestyles. They rely on thick insulating fur or feathers as well as burrows, dens, or nests to keep their body temperatures high.
Mammals such as the Canada lynx, snowshoe hare, and wolverine are superbly adapted to life in winter mountains. Pikas, as well as many rodents and shrews, stay awake beneath the snow, close to the ground or in burrows. Room to move beneath the snow is created when heat from the soil melts enough snow to create an open layer of air known as the subnivean zone. Voles, moles, and shrews spend the winter in the subnivean zone. Foxes and weasels dig through the snow to catch the small, burrowing mammals. Squirrels such as Douglas’ squirrel, the western gray squirrel, and the flying squirrel stay awake and either forage or rely on stores of seeds and other plant material.
Birds that remain active in the mountains include ravens, gray jays, Stellar’s jays, mountain chickadees, and woodpeckers.
The mountains are pretty quiet in the winter. Many animals are either gone or hibernating. But life goes on up in the Cascades and Olympic mountains, all year long. For those of us willing to brave the cold, there is plenty of wildlife to experience in the winter landscapes of our northwest mountains.